Check the Error Log view. You may find a stack trace that is more helpful.

But, the likely cause of the error is that JET failed to load your Project
as a bundle into your workbench. This can happen for any of the following
reasons (and probably more):

1) you JET project refers to other projects in the workspace. JET will
attempt to load the JET project, but not any referenced projects. Given that
in another thread you state the your JET templates co-exist with some GMF
code, this may be the problem.
2) you are using a 1.4 JRE, but have the Eclipse Java compiler set to
compile the code to Java 5. You'll see an UnsupportedClassVersionError in
the stack trace.